Signatures of inhomogeneous dark matter annihilation on 21-cm

TL;DR

The paper investigates how structure formation induces an inhomogeneous DM annihilation rate, boosting local energy deposition into the IGM and imprinting distinctive fluctuations in the 21-cm signal. By decomposing the DM injection into contributions from collapsed halos and uncollapsed IGM and introducing a boost factor $B(x)$ tied to the conditional halo mass function, the authors quantify the resulting impact on the 21-cm brightness temperature $T_{21}$ and its power spectrum using HyRec and 21cmFAST with deposition-channel transfer functions $ig(\mathcal{T}^{s}_{ m c}ig)$. For a representative $e^+e^-$ channel with $m_ ext{χ}=100$ MeV and $ig<\sigma vig>/m_ ext{χ}\sim10^{-27} ext{ cm}^3 ext{s}^{-1} ext{GeV}^{-1}$, inhomogeneous annihilation can enhance the 21-cm power spectrum by up to a factor of $ oughly 130$ in the redshift interval $11\, extless z \, extless 16$, offering a potential SKA detection. The study highlights that localized energy deposition from low-energy annihilation products yields pronounced inhomogeneities aligned with the density field, while noting limitations such as the use of a spatially averaged deposition efficiency and truncation of deposition at $z<11$, and it discusses the need to extend the analysis to broader DM masses and channels. Overall, this work provides a framework to isolate DM-induced inhomogeneities in the 21-cm signal, underscoring the prospect of using SKA measurements to constrain or detect DM annihilation during cosmic dawn.

Abstract

The energy released from dark matter (DM) annihilation leads to additional ionization and heating of the intergalactic gas, impacting the hydrogen 21-cm signal during the cosmic dawn. The dark matter annihilation rate scales with its density squared and becomes inhomogeneously boosted with structure formation. This paper examines the inhomogeneity in DM annihilation rate induced by the growth of DM halo structures, and we show that this effect can significantly amplify the spatial fluctuations in temperature and ionization fraction of the gas. Consequently, the fluctuations in the 21-cm brightness temperature may also be enhanced. We showcase these effects for a DM mass of 100 MeV annihilating into $\rm{e}^-\rm{e}^+$ at a rate of $\left<σv\right>/m_χ\sim 10^{-27} {\rm cm^3 s^{-1} GeV^{-1}}$, which is consistent with current constraints set by the cosmic microwave background. We find that, compared to the homogeneous calculations, inhomogeneous annihilation can enhance the 21-cm power spectrum by up to a factor of 130 over the scales of $k \in [0.05, 3]\ {\rm{Mpc^{-1}}}$ at redshifts $11-16$. Such signatures could potentially be detected by upcoming radio observatories such as the Square Kilometer Array telescope.

Figures (5)

• Figure 1: Lightcone simulations of the inhomogeneous boost factor (top) and the density field ($\delta$).The third to eighth panels visualize the evolution of gas temperature $T_{\rm k}$, ionisation fraction $x_{\rm e}$ and 21-cm temperature $T_{21}$ in presence of inhomogeneous/homogeneous boost factor, both using $\left<\sigma v\right>/m_\chi = 10^{-27} {\rm cm^3 s^{-1} GeV^{-1}}$ and $m_\chi = 100\ {\rm MeV}$. The panels labeled with Inhomogeneous Boost correspond to the scenario in which DM annihilation products have relatively low energy and are therefore absorbed locally. If DM injects high-energy particles that have a long mean free path before absorption, the effect of inhomogeneous boost factor would be washed out. This corresponds to the panels labeled with homogeneous boost, for which the boost factor is added uniformly using its spatially averaged value. Note that the growth and fluctuation of the boost factor trace those of the density field, and the panels with inhomogeneous boost factor exhibit more fluctuations than those with homogeneous boost factor. The power spectrum shown in Fig. \ref{['Fig_Power_T21']} provides more quantitative comparison of these inhomogeneities.
• Figure 2: Left--global average of boost factor $B$ (black) and collapse fraction $f_{\rm coll}$ (red). Middle and right--power spectrum for $B$ (black) and density contrast (red, scaled by the mean boost factor $\bar{B}$).
• Figure 3: Evolution of spatially averaged $T_{\rm k}$ (top) and $T_{21}$ (bottom). The black and red lines represents the Fiducial and IHM simulations respectively, results for HMG are found to be the same as that in IHM simulation and are thus not shown here. The blue line on top panel shows the CMB temperature. The legend applies to both panels.
• Figure 4: Top: the 21-cm power spectrum $\bar{T}^2_{21}\Delta_{21}^2$ at several characteristic scales and redshifts. The black, red and blue lines represent the Fiducial, HMG and IHM simulations respectively. The left and middle panels show $\bar{T}^2_{21}\Delta_{21}^2$ at $k = 0.08 {\rm Mpc}^{-1}$ and $k = 0.18 {\rm Mpc}^{-1}$ respectively, and the right panel shows results for $z=13.5$ (solid) and $z=18$ (dashed). Both IHM and HMG simulations assume the same DM injection parameters of $\left<\sigma v\right>/m_\chi = 10^{-27} {\rm cm^3 s^{-1} GeV^{-1}}$ and $m_\chi = 100\ {\rm MeV}$, in HMG simulation $\bar{T}^2_{21}\Delta_{21}^2$ can be enhanced by orders of magnitude compared to that of IHM. Such difference can potentially be detected by the SKA telescope with 2000 hours of observation time (green solid curve) Sitwell:2013fpa. Bottom: $\bar{T}^2_{21}\Delta_{21}^2$ at different scales and redshifts, the left and middle panels correspond to the HMG and IHM simulations, respectively. The right panel shows their relative difference, in regions outside the black contour line, $\bar{T}^2_{21}\Delta^2_{21}$ for the IHM simulation is enhanced relative to the HMG case. For visual illustration, the color bar is truncated around 10, and the actual relative difference can be much higher in some regions.
• Figure 5: Spatial slice of our simulation lightcones for $T_{21}$ (left) and $T_{\rm k}$ (right) at $z = 13.5$, corresponding to the redshift when the enhancement of 21-cm power spectrum in IHM (bottom) relative to the HMG (top) simulation is maximum. To highlight the fluctuation patterns, in $T_{\rm k}$ panels we truncate the color bar for $T_{\rm k} > 100$ K.